Clock synchronous multicarrier demodulator for multi-frequency TDMA communications satellites

Some design and implementation aspects of an eight-channel 2.048-Mb/s multicarrier demodulator for multifrequency TDMA (time division multiple access) systems, requiring assured network synchronization and the likely absence of preambles in data bursts, are reported. The 32 voice channels in a 2-Mb/s primary group are assigned an equal number of time slots in the frame; timing (symbol) synchronization permits each slot to be occupied by different users from frame to frame if needed by the overall network efficiency. The study phase started with the requirement of a fully digital demodulator, substantially exploiting multistage and block approaches for the demultiplexer. The latter approach was selected for detailed design and implementation, utilizing a bank of polyphase filters followed by an eight-point fast Fourier transform. Carrier recovery-not trivial because of the absence of preambles-can be implemented by either exploiting the inherent frame-to-frame coherence in a PLL (phase locked loop)-based scheme or a nonlinear phase estimation algorithm originally proposed by Viterbi